The present invention generally relates to increasing data throughput or quality in a wireless communication system and, more particularly, to systems and methods involving adaptive modulation and FEC coding schemes.
The growth of commercial communication systems and, in particular, the explosive growth of cellular radiotelephone systems have compelled system designers to search for ways to increase system capacity without reducing communication quality beyond consumer tolerance thresholds. At the same time usage of mobile communication equipment for transmission of data rather than speech has become increasingly popular by consumers. The ability to send and receive electronic mail and to use a web browser to obtain world-wide-web access is frequently discussed among services that will be used more and more in wireless communication systems. In response to this, communication system designers search for ways to efficiently transfer data information to and from mobile users.
There are fundamental differences between requirements for data communication and e.g., speech communication. For example, delay requirements are higher for speech, which is a real time service, and the error requirements are higher for data communication, while the delay constraints are lower. The use of packet data protocols, which are more suitable for transmission of data than circuit switched protocols, starts to find its way into cellular communication systems. Packet service integration in both GSM cellular systems as well as DAMPS cellular systems is presently being standardized.
Today, GSM systems provide a circuit switched data service, which can be used to interconnect with external data networks. The circuit switched data service is used for both circuit switched as well as packet switched data communication. To make packet switched data communication more efficient, a new packet switched data service called GPRS, (General Packet Radio Services) is introduced. GPRS will support both connectionless protocols (e.g., IP) as well as a connection-oriented protocol (X.25). One of the advantages with a packet switched data communication protocol is that a single transmission resource can be shared between a number of users. Thus, in the case of e.g., a GSM cellular system, a timeslot on a radiofrequency carrier can be utilized by several mobile users for reception and transmission of data.
GPRS is a GSM service and parts of the GSM infrastructure will be used. The GSM communication system is described in the European Telecommunication Standard Institute (ETSI) documents referenced below.
The aims of introducing a packet data protocol in cellular systems are mainly to be able to support high data rate transmissions and at the same time achieve flexibility and efficient utilization of the radio frequency bandwidth over the air interface. The concept of GPRS can be implemented in any TDMA system and extended also for multicarrier/multislot protocols, where a single user is allowed to occupy more than one transmission resource simultaneously.
In order to provide various communication services, a corresponding minimum user bit rate is required. For example, for voice and/or data services, user bit rate corresponds to voice quality and/or data throughput with a higher user bit rate producing better voice quality and/or higher data throughput. The total user bit rate is determined by a selected combination of channel FEC coding and modulation, in combination with the number of transmission resources allocated for a user. For example, in a TDMA system the number of transmission resources corresponds to the number of timeslots.
Different communication systems use different FEC coding schemes to communicate voice or data information. The modulation techniques include, for example, Gaussian Minimum Shift Keying (GMSK), Quadrature Phase Shift Keying (QPSK) and different levels of Quadrature Amplitude Modulation (QAM). Combinations of the FEC coding and modulation schemes may provide different user bit rates. Usually a high user bit rate combination implies a need for a channel with low interference level, whereas a low user bit rate combination has a lower requirement on interference. A conventional term used to indicate tolerance to high or low interference levels is robustness. The robustness is usually indicated by a carrier-to-interference level measure (C/I ratio). This ratio expresses the relation between the signal strength of the useful signal on a radio channel and the interference level on that channel. Other indications of robustness may also be applied, e.g., carrier to noise ratio (C/N). For brevity, C/I is used throughout this text.
Conventionally, a communication system operates using a single modulation scheme and a single FEC coding scheme with a certain rate for transmission of information under all radio channel conditions. However, as the number of mobile users increases, the radio channel conditions become more diverse in different areas and at different times. The interference from several simultaneous transmissions may severely degrade performance for a certain combination of modulation and FEC coding with a high user bit rate, while at other instants, the interference level may be low enough to allow for such a combination. More recently, however, dynamic adaptation of modulation and FEC coding scheme combinations used for transmission in radiocommunication systems has been considered as an alternative. Depending upon the radio channel conditions, a suitable combination with a sufficient robustness may be applied and an optimal user bit rate may be provided. Switching between different combinations of modulation and FEC coding during transmission is called link adaptation and this feature is being considered for future radiocommunication systems and as an improvement for existing systems. An example of a communication system employing multiple modulation schemes is found in U.S. Pat. No. 5,577,087. Therein a technique for switching between a higher level QAM and QPSK is described. The decision to switch between modulation types is made based on quality measurements, however this system employs a constant user bit rate which means that a change in modulation also requires a change in channel user bit rate, e.g., the number of timeslots used to support a transmission channel.
Thus, it is possible to consider providing different link protocols, i.e., combinations of different types of modulation and channel FEC coding in one system, which link protocols are adaptively changed for a particular radio channel condition, i.e., radio link. For example, two link protocols may exist in a system, where one has a lower robustness (i.e., using less FEC coding and/or a less robust modulation) and one has a higher robustness (i.e., using more FEC coding and/or more robust modulation). The link protocol being used by the system at any given time is referred to herein as a realization.
Link protocol performance can be characterized by Bit Error Rates (BER) as a function of signal quality, e.g., C/I. A less robust link protocol is typically sensitive to interference, i.e., needs high C/I to achieve low BER. On the other hand, a less robust protocol also provides a higher user bit rate. Since the C/I varies between links in a system, different protocols are suited for different links. A link adaptation algorithm adaptively selects the realization that maximizes the throughput or user perceived quality. Since the C/I varies with time, the realization should be reevaluated continuously. The time between these realization reevaluations is referred to herein as the update interval.
When a transmission link is initially established, there may be no or limited previous information regarding the quality of the radio channel. It is therefore difficult to select the correct realization from the beginning, i.e., an initial realization that maximizes the throughput on that particular radio channel. Thus, it is likely that a non-optimal (e.g., fixed/default or based on inaccurate link quality estimates) realization will be used as an initial choice.
It is known in the literature that evaluating the performance of a particular realization and selectively change to another realization, i.e., link protocol, has occurred at fixed update intervals. However, Applicants have recognized that using a fixed update interval for link adaptation may lessen throughput because, for example, the initial realization is frequently non-optimal. Thus innovative techniques for link adaptation techniques are needed.